1. Field of the Invention
This inventions relates to optical disc apparatuses, and more particularly to an optical disc apparatus which is arranged to read desired data out of an optical disc having audio data areas and CD-ROM areas coexisting thereon.
2. Description of the Prior Art
There are optical discs formed with a CD-ROM area having computer code data and an audio data area in a coexisting manner. In an optical disc apparatus loaded with such an optical disc, the optical disc is frequently switched of rotational speed or reproducing speed depending upon data to be read out. That is, the audio data is reproduced at a normal speed (1-times speed), whereas the computer code data is at a speed of a predetermined number times higher the normal speed (e. g. an 8-times speed).
The disc reproduced signal (reproduced EFM signal), read out of an optical disc through an optical pick-up, is configured by a plurality of pattern data different in pulse width (3T-11T). Accordingly, there is a necessity of accurately discriminating the pulse width by reproducing a clock that is synchronous with the reproduced EFM signal. The reproduced EFM signal is synchronously processed by a PLL circuit, wherein the reproduced EFM signal is compared with an oscillation clock of a VCO (voltage controlled oscillator) to control the VCO oscillating frequency in response to an error difference (voltage) between them. Consequently, the optical disc apparatus, requiring frequent changeover of reproducing speed, is structured to vary a PLL characteristic to meet with a reproducing speed, for example, by adjusting the frequency-dividing ratio of a variable frequency divider for dividing the VCO output.
Meanwhile, the audio data can be read only at a certain given frequency (normal speed), whereas the computer code data or the like can be read out over a comparatively broad speed range. That is, it is possible to read data, depending upon a frequency pulling range of the PLL circuit, from an optical disc (rotating, e.g. at a 5-times speed) before reaching an 8-times speed, as shown in FIG. 5. Nevertheless, it is impossible to read data during a time period (T) before the reproducing speed reaches a speed 5 times higher than the normal speed, as will be understood from FIG. 5. To this end, there has been a problem of worsening in a response characteristic to a data read request outputted from a host computer.